1. Field of the Invention
The invention relates in general to protective relays, and more specifically to protective relays for protecting high-voltage transmission line sections in a three-phase electrical power system.
2. Description of the Prior Art
Pilot protection for high-voltage electrical transmission lines identifies faults on the protected line section by using communication channels for relays to compare system conditions at the terminals or ends of the line section. For economic reasons, power line carrier or microwave pilot channels are used for long transmission lines, and pilot-wire relaying is used for protecting short transmission line sections, i.e., usually about ten miles, or less. Pilot-wire relaying is very important, as in short high-voltage lines, discrimination is difficult with distance type-relays, making pilot relaying the only practical method of discrimination not based on time delay.
The electromechanical pilot-wire relays of the prior art require a continuous metallic circuit between the terminals of the protected line section. A composite sequence filter at each terminal converts the three-phase currents into a single-phase voltage, and the single-phase voltage is applied to the pilot-wire pair for comparison with the other end single-phase voltage. The protective relay at each terminal is an electromechanical relay which includes an operating coil and a restraint coil. The operating and restraint coils are connected in the pilot-wire system such that with normal through current in the protected line section, the polarities of the two single-phase voltages developed by the composite sequence filters cause more current to flow in the restraint coils than in the operating coils. An internal fault, i.e., a fault in the protected section, causes one of the single-phase voltages to shift its phase angle and then the instantaneous polarities are such that more current flows through the operating coils than through the restraint coils, causing the relays to simultaneously trip their associated circuit breakers to clear the protected transmission line section.
The comparison and decision making element of the electromechanical pilot-wire is a polar unit which includes the operating and restraint coils. A unique feature of this relay is the interaction of the 60 Hz quantities on the pilot wire, which interaction is a part of the decision making process of the electromechanical relay system.
The electromechanical pilot-wire relay system securely, reliably and economically provides high speed simultaneous clearing of the line terminals of a protected transmission line section due to a fault in the protected section. Continuous, end-to-end metallic circuits, for use in pilot-wire relaying, which are typically leased from a telephone company, are becoming increasingly more difficult to obtain, or even retain. Thus, there is a need for a new and improved pilot relaying system which has the desirable characteristics of discriminatory in-phase and out-of-phase tripping, and the economic attractiveness of the pilot-wave relaying system, but which does not require that the communication link be a continuous metallic circuit.
U.S. Pat. No. 4,275,429, which is assigned to the same assignee as the present application, discloses such a relay which may use a non-continuous communication channel, such as a low grade, uninterruptible dedicated telephone line. A dedicated telephone line maintains an open channel at all times, but it has relatively low current capability, and it is not a continuous metallic circuit, as it may include amplifiers and switchboards in the circuit. The relaying system of this patent is also suitable for use with an optical link, i.e., a light pipe, microwave, or power line carrier.
The evaluation function in the relaying system of U.S. Pat. No. 4,275,429 is completely different than the functioning of the decision element of the electromechanical relay. It provides a vector or phasor combination of the local and remote signals, and it provides a true decision signal when the result exceeds a predetermined magnitude. It also determines the phase angle between the local and remote signals, and it provides a true decision signal when the angle is in a predetermined range. If these two decision signals are simultaneously true, it indicates a fault in the protected line section, and each relay will generate a trip signal for its associated circuit breaker. While this evaluation circuit provides excellent results, market acceptance of a solid-state relay which replaces an electromechanical relay is more rapidly obtained when its functioning can be explained with reference to the functions of the electromechanical relay. Thus, it would be desirable to provide a new and improved solid-state relay for pilot protection which has the ability to use a non-continuous communication channel, but which functions broadly in a manner similar to the electromechanical pilot-wire relay. Since the continuous pilot wire is not used, the solid-state relay must modulate the 60 Hz quantity on a channel of high frequency, and thus the hereinbefore mentioned interaction of the quantities obtainable with the pilot wire does not occur, and it must be replaced by other means if the solid-state relay is to functionally compare with the electromechanical relay.